FIG. 5 shows an example of conventional RF drive circuit of MR apparatus. The RF drive circuit 51 has its input terminal IN supplied with an RF pulse signal having a prescribed envelope. The RF pulse signal is conducted through a distributor 2 and fed to a variable-gain amplifier 3. The variable-gain amplifier 3 operates in unison with a power amplifier 4 to amplify the RF pulse signal to produce a drive voltage of a prescribed voltage level, which is fed to a quadrature hybrid 5. The quadrature hybrid 5 produces a 0.degree.-component drive voltage and a 90.degree.-component drive voltage from the input drive voltage, and applies the resulting drive voltages to a 0.degree.-port and 90.degree.-port of an RF coil 6. By being energized by the two-component drive voltages, the RF coil 6 generates an RF revolving magnetic field in its internal space.
A voltage monitor means 52 monitors the output voltage of the power amplifier 4 and feeds the result to an envelope extracting circuit 53, which extracts the envelope of the monitored device voltage. Another envelope extracting circuit 11 connected to the distributor 2 extracts the envelope of the RF pulse signal provided by the distributor 2.
A comparator 12 compares the envelopes of the RF pulse signal and output voltage of the power amplifier 4, and controls the gain of the variable-gain amplifier 3 in accordance with the comparison result so that the output voltage of the power amplifier 4 has the prescribed value regardless of the load condition of the coil (the impedance of the patient who lies in the internal space of the RF coil 6).
The foregoing conventional RF drive circuit 51 merely controls the output voltage of the power amplifier 4 to the prescribed level, but it does not control the balance of the voltages applied to the 0.degree.-port and 90.degree.-port of the RF coil 6. Therefore, if the RF coil 6 loses the impedance balance due to different impedances of the 0.degree.-port and 90.degree.-port, the quadrature balance, i.e., the condition of RF revolving magnetic field generation, varies, resulting in a varying flip angle for spin induction.